This encapsultates the different implementations and allows us to
compile debug.cpp in the common libraries without link errors.

BUG=angleproject:513

Change-Id: I16dc4c666fb4266ee5146d64d77eb9925c7584a8
Reviewed-on: https://chromium-review.googlesource.com/256450Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>